paper

Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)

arXiv:1509.04230 · doi:10.21468/SciPostPhys.3.4.026

Abstract

One of the most fundamental properties of an interacting electron system is its frequency- and wave-vector-dependent density response function, $I ¨ (q, I ¨ )$ . The imaginary part, $I ¨ ^{''} (q, I ¨ )$ , defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. $I ¨ $ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure $I ¨ (q, I ¨ )$ at the meV energy scale relevant to modern elecronic materials. Here, we demonstrate a way to measure $I ¨ $ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as "M-EELS," allows direct measurement of $I ¨ ^{''} (q, I ¨ )$ with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi $_{2}$ Sr $_{2}$ CaCu $_{2}$ O $_{8 + x}$ (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control.

26 pages, 10 sections, 7 figures, and an appendix